Improving precision in regional scale numerical simulations of groundwater flow into underground openings

Abstract

Dewatering is a common engineering practice to secure the accessibility during the construction and operational phases of tunnels. It is thus important to accurately estimate the amount of groundwater flow into underground openings for the design and safe operation of tunnels. Numerical models need to be used to estimate groundwater flow into openings for given heterogeneity and regional hydrologic boundary conditions typically by assuming that the atmospheric pressure is maintained along open wall faces. As the scale of openings can be small as tens of centimeters, while models need to incorporate the regional hydrologic conditions, it is often practically impossible to explicitly represent the three-dimensional (3D) geometry of openings within numerical models. For the safety assessment of deep geological repositories of spent nuclear fuels, for example, complex configurations and geometry of various tunnels such as construction, access, ventilation, and deposition tunnels often need to be approximated as one-dimensional (1D) lines in 3D models. The application of a simple Dirichlet boundary condition along a set of tunnel nodes may yield inaccurate solutions due to geometric simplifications and coarse discretization. This study derives an appropriate boundary condition that can be applied to 1D tunnel segments to improve the accuracy when simulating inflow into openings. A third-type tunnel boundary condition is suggested to correct the difference between known wall pressure and the pressure simulated at tunnel nodes. Improvement in the precision of the solutions is demonstrated by comparing the numerical solutions using the new boundary condition with analytic solutions, and with numerical solutions when the 3D tunnel geometry is explicitly simulated. It is also shown that the formulation can be easily extended to incorporate the high permeability excavation damaged zone and the low permeability grouted zone in the tunnel vicinity. The applicability of the tunnel boundary condition is demonstrated using an example of a hypothetical deep geologic repository system consisting of various types of tunnels located in a hypothetical crystalline coastal aquifer.